1. Field of the Invention
This invention relates to solid state light emitting devices and in particular to light emitting devices based on LEDs (light emitting diodes). More particularly, although not exclusively, the invention concerns light emitting devices that are operable from high voltage (typically 110 or 220V) power sources. Moreover the invention concerns AC-drivable light emitting devices that generate white light with a CRI (Color Rendering Index) of at least 90.
2. Description of the Related Art
White light generating LEDs, “white LEDs”, are a relatively recent innovation and offer the potential for a whole new generation of energy efficient lighting systems to come into existence. It is predicted that white LEDs could replace incandescent, fluorescent and compact fluorescent light sources due to their long operating lifetimes, potentially many 100,000 of hours, and their high efficiency in terms of low power consumption. It was not until LEDs emitting in the blue/ultraviolet part of the electromagnetic spectrum were developed that it became practical to develop white light sources based on LEDs. As taught, for example in U.S. Pat. No. 5,998,925, white LEDs include one or more phosphor materials, that is photo-luminescent materials, which absorb a portion of the radiation emitted by the LED and re-emit radiation of a different color (wavelength). Typically, the LED chip or die generates blue light and the phosphor(s) absorbs a percentage of the blue light and re-emits yellow light or a combination of green and red light, green and yellow light or yellow and red light. The portion of the blue light generated by the LED that is not absorbed by the phosphor is combined with the light emitted by the phosphor to provide light which appears to the human eye as being nearly white in color.
Since LEDs are intrinsically direct current (DC) devices and will only pass an electrical current in a single direction they have typically been driven by low voltage (e.g. 3.5V for gallium nitride (GaN) LEDs) DC sources.
However in lighting applications it is desirable to be able to operate white LEDs directly from a high voltage (110/220V) AC mains power supply without the need for expensive power supplies and driver circuitry.
U.S. Pat. No. 6,957,899 to Jiang et al. disclose a single-chip integrated LED 1 that is for direct use with a high voltage (110V/220V) AC power source. The LED 1 comprises a plurality of series-connected LEDs arranged in two strings (arrays) 14, 18 (FIG. 1). The strings 14, 18 are connected in parallel with the LEDs in opposite polarity in a half-wave rectifier configuration such that the LEDs are self-rectifying. A sufficient number of LEDs (e.g. twenty eight per string for 110V operation and fifty five per string for 220V operation) is provided in each string to drop the total source voltage across the LEDs. During the positive half of the AC cycle one string of LEDs is forward biased and energized, while the other string is reverse biased. During the negative half of the AC cycle, the other string of LEDs is forward biased and energized, while the first string is reverse biased and not energized. Thus the strings are alternately energized at the frequency of the AC supply (50-60 Hz) and the single-chip LED appears to be constantly energized. The single-chip LED is formed by epitaxially depositing layers of n-type semiconductor material, optically active layers and p-type semiconductor material in succession to define individual LEDs on a single wafer. Adjacent LEDs are interconnected by depositing conducting layers between individual LEDs. Although such a fabrication is compact it has a disadvantage that since only one LED string is energized at a time the arrangement has only a 50% payload.
At present, the yield rate of GaN (gallium nitride) LED chips is unable to guarantee a uniform brightness or a 100% light emitting efficiency of each LED chip causing a non-uniform light emission. Additionally if there is a surge in the drive voltage the LED chips in the light emitting device can be burned out, and the reliability can become an issue.
As shown in FIG. 2, Taiwanese Patent Application Publication No. 200826320, to Lin et al., teach an AC-drivable light emitting device that includes at least one correction circuit 20 that operates as a temperature and voltage compensation circuit. In operation the correction circuit absorbs power surges to protect the light emitting diode 21.
United States patent application Publication No. US 2008/083929, to Fan et al., disclose a high voltage AC/DC drivable LED device with an integrated protection mechanism. FIG. 3 is a schematic cross-sectional view of the device and shows a monolithically integrated current-limiting resistor 30 on the LED chip. Whilst the current limiting resistor provides protection to the device it dissipates energy in the form of heat generation reducing the overall luminous efficacy of the device.
United States patent application Publication No. US 2008/218098, to Lee et al., teach a light emitting device with a monolithically integrated bridge rectifying circuit. FIG. 4 is a schematic plan view of the light emitting device of Lee et al. showing integration of the bridge rectifying circuit 40 around the peripheral edges of an array of serially-connected LEDs 41. It is claimed that the rectifying circuit 40, which is composed of diodes, improves reliability of operation the device and luminance. However, since the bridge rectifying circuit 40 is directly formed on the LED substrate it is not possible to produce LEDs with different light emission wavelengths on the same substrate and such a device still has the drawback that the diodes of the rectifier circuit dissipate energy without contributing to light emission of the device.
A need exists for high voltage drivable solid state light emitting devices that in part at least overcome the limitations of the known devices.